1. Field of the Invention
The present invention relates to a motor-operated valve used by being assembled into an air conditioner, a refrigerator, or the like and, more particularly, to a motor-operated valve capable of controlling the flow rate of a refrigerant accurately to a minute amount even in the case where a high-pressure refrigerant is used.
2. Description of the Related Art
For example, Patent Reference 1 describes a conventional general motor-operated valve. This motor-operated valve has a mechanism in which a rotor disposed on the inside of a can is rotationally driven by a stator provided on the outside of the can, and the rotational motion of rotor is converted into linear motion by a screw mechanism, by which the valve opening of a valve element with respect to a valve seat is controlled.
[Patent Reference 1] Japanese Patent Laid-Open No. 2001-50415
In the conventional motor-operated valve as described above, the flow rate of refrigerant is determined by the effective opening area of a valve port, namely, a lift amount of the valve element from the valve seat, and the lift amount of the valve element is determined by the pitch of the screw mechanism and the number of revolutions of the rotor (the number of steps of the stepping motor).
Generally, in the motor-operated valve of this type, in order to achieve a desired maximum flow rate, the maximum lift amount (fully opened state) of valve element, in other words, the maximum number of steps of the stepping motor is determined in advance. That is to say, stator energization control is carried out so that the rotor is rotated, for example, five turns at a maximum. In this case, in the case where the pitch of the screw mechanism is set, for example, at 0.6 [mm], if the rotor and a valve stem holder rotate one turn, two turns, three turns, four turns, or five turns, the lift amount of the valve element changes to 0.6, 1.2, 1.8, 2.4 or 3.0 [mm] at intervals of 0.6 [mm].
However, in particular, in a motor-operated valve (used as an expansion valve) used for a refrigerator or the like, the control in the prior art is rough although the controlled refrigerant flow rate is very low. Especially when carbon dioxide (gas) or the like are used, the pressure of refrigerant must be increased (about 10 times as compared with the conventional example). In the case where the high-pressure refrigerant is used, if the valve element is lifted at intervals of 0.6 [mm], there arises a problem in that a change in flow rate of the refrigerant becomes too great.
As one measure for solving this problem, it is conceivable, for example, that the diameter of the valve port or a valve angle (effective opening area) is decreased. However, the decrease in effective opening area has a limitation, and only such a measure cannot solve the above-described problem completely.
Also, as another measure, it is conceivable that the pitch of the screw mechanism is decreased (for example, from 0.6 [mm] to 0.2 [mm]). However, if the pitch of the screw mechanism is decreased, a highly accurate fabrication is required, and thus the fabrication cost increases. In addition, there arises a problem in that the lift amount per one rotor rotation is small, so that the contact and separation of a movable stopper with and from the fixed stopper, these stoppers being used to regulate rotation, are not accomplished properly.